It's likely that the cost of renting a costume is going to outweigh the value but stuff you 'blag' or make can be just as effective. Some charities have mascot costumes which they use for awareness as well as fundraising (Monty the Duck for the Meningitis Trust springs to mind) and they can be fantastic to collect in if you are anywhere where there are going to be families with kids. A friend of mine procured a chicken costume a while back for a few quid as it was ex-rental and that saw him through for quite a while.
*** Cancer Campaign do an annual 'Wear it Pink' collection in London and they encourage everyone to dress in pink which serves the double benefit of being very visible and also the charity colour (and the one most associated with *** cancer).
It's also worth making sure that you keep the charity branding visible, and if possible anything to reinforce the fact that you are a volunteer (if that's the case). The general public have become quite jaded now with the streets flooded with commission paid ch'muggers so a good example of clothing would be Help the Aged's t-shirts which are bright yellow, have the logo emblazoned on the front and have the words "Volunteer Collector" on the back in large letters.
Personally I prefer to collect money in the Tigger (of Winnie the Pooh fame) costume that I was bought as a present a few years back. They sold them briefly in the US in adult sizes for about $40 and it's lasted me a good seven years and raised a fair few quid as it seems to appeal to people or all ages although I seem to get a lot of people who confuse me with Tony the Tiger.
